A) Field of the Invention
This invention relates to a liquid crystal display device and a process for producing the same.
B) Description of the Related Art
For a vertical alignment type liquid crystal display device in which liquid crystal molecules in a liquid crystal layer are aligned vertically or substantially vertically to the substrate, its transmittance in the absence of applied voltage is very low and almost equivalent to the optical characteristics of two polarizers placed substantially in crossed-Nicol. If an optical compensation plate having negative optical anisotropy with an appropriate parameter is introduced between the liquid crystal layer and one or both of the two polarizers sandwiching it, better viewing characteristics can be obtained.
A method of performing viewing angle compensation by using an optical compensation plate having negative uniaxial optical anisotropy, so-called (negative) C plate, as a viewing angle compensation plate is described in Japanese Unexamined Patent Publication (Kokai) No. SHO-62-210423. By this method, optical anisotropy of a vertically aligned liquid crystal layer can be canceled to eliminate viewing angle characteristics in the absence of applied voltage. However, light leakage takes place when the viewing angle is changed, particularly when observing from an azimuthal angle of approximately 45° from the absorption axis of one of two substantially crossed-Nicol polarizers. This phenomenon occurs because there exist viewing angle characteristics of the polarizers themselves and it is known that this phenomenon causes deterioration of viewing angle characteristics.
As a method to solve the problem, Japanese Patent 2047880 proposes the use of an optical compensation plate having negative biaxial optical anisotropy (biaxial film). In addition, Japanese Unexamined Patent Publication (Kokai) No. 2000-131693 describes examples of particularly effective parameters for biaxial films. By adopting a liquid crystal display device using a biaxial film, deterioration of viewing angle characteristics due to polarizers, which poses a problem when a C plate is used, can be eliminated, allowing a black display feature almost equivalent to that observed from the front to be realized even if a liquid crystal display device is observed from an azimuthal angle of approximately 45° from the absorption axis of a polarizer.
The present inventor has proposed a vertical alignment type liquid crystal display device that achieved uniform mono-domain alignment by means of rubbing treatment in Japanese Unexamined Patent Publication (Kokai) No. 2005-234254. The above-mentioned viewing angle compensating method can also be applied to this liquid crystal display device.
However, even in a vertical alignment type liquid crystal display device using a biaxial film, when observed from a large polar angle at an azimuthal angle of 45° with respect to the absorption axis of a polarizer (large polar angle with respect to the normal direction of a polarizer), leaked light of blue, violet, or other colors is sometimes observed in the absence of applied voltage. When a C plate is used, leaked light having a color is not observed even if observed from a large polar angle at an azimuthal angle of 45° with respect to the absorption axis of a polarizer although light leakage is large because the viewing angle characteristics of the polarizer cannot be compensated.
As a viewing angle compensation plate for a vertical alignment type liquid crystal display device, biaxial films capable of reducing light leakage have been widely used because numerical excellence of viewing angle characteristics is prioritized, and products available on the market include biaxially stretched films formed of norbornene-based cyclic olefin material, and stretched TAC films produced by stretching TAC material used as base. On the other hand, for optical films having optical characteristics equivalent to a C plate, the availability on the market has been rapidly reduced and they are difficult to obtain.
In Japanese Unexamined Patent Publication (Kokai) No. 2008-233412, the present inventor has proposed a vertical alignment type liquid crystal display device in which random recesses and protrusions are formed on the surface on the liquid crystal layer side of at least one substrate. By making use of the phenomenon that in a vertical alignment type liquid crystal display device using a biaxial film, the color tone of leaked light changes variously in the visible light wavelength range with a change in the thickness of the liquid crystal layer when observed from a large polar angle at an azimuthal angle of 45° with respect to the absorption axis of a polarizer, many regions with different liquid crystal layer thicknesses are distributed at random in the liquid crystal layer and various colored lights are mixed to obtain a neutral-color appearance so that the transmissivity of the leaked light is reduced significantly while the color tone of the leaked light is maintained equivalent to that for the conventional C plate compensation.
For the liquid crystal display device described in Japanese Unexamined Patent Publication (Kokai) No. 2008-233412, random protrusions and recesses need to be formed on at least one of the two substrates. For their formation, though some methods including the use of chemical etching of a glass substrate (frost processing), the use of physical etching (sand blast processing), and coating of a substrate with a hard coat agent after spraying particles on the substrate have been proposed, all of them require high cost. Moreover, because these methods assume that treatment is performed on the whole surface of a mother glass, there are cases where protrusions and recesses are formed also on an external extraction electrode (electrode terminal) that connects a liquid crystal display device to an external driving circuit and protrusions and recesses are formed even on the seal frame region that bonds two substrates, resulting in a danger of the lowering of the production yield. In addition, because presently manufactured liquid crystal display devices adopt multiple electrode patterns in which a plurality of liquid crystal display devices are arranged on a surface of a mother glass before two glass substrates are assembled, the alignment marks for assembling can become blurred due to light scattering on the surfaces of the stacked substrates if protrusions and recesses are formed on the whole surface of the mother glass, affecting the production yield adversely. In the appearance observation of a liquid crystal display device, display non-uniformity may be observed in any of the light display regions, dark display regions, and regions where no voltage is applied (background region) because protrusions and recesses are arranged in random. This is considered to be caused by light scattering due to the refractive index difference between protrusions or recesses on the substrate and the liquid crystal layer.